Vehicles powered with diesel engines have to be equipped with lean exhaust aftertreatment devices such as a urea-based Selective Catalytic Reduction (SCR) catalyst to reduce NOx emissions. The urea-based SCR catalyst requires the injection of urea to provide ammonia (NH3) as a reductant for NOx reduction. The ammonia generation from urea decomposition follows mainly two steps, as shown below:NH2—CO—NH2(gas or liquid)=NH3(gas)+HNCO(gas)  (1)HNCO (gas)+H2O(gas)=NH3+CO2(gas)  (2)
The first step (1) is a thermal decomposition reaction and the second step (2) is a hydrolysis reaction. The thermal decomposition of urea is slow and is the rate limiting step at a temperature below 300° C. Therefore, when the exhaust temperature is below 300° C., the spray of urea solution to the SCR catalyst may be deposited mostly as urea related compounds on the SCR without being fully decomposed.
Diesel exhaust gas temperature may be low (e.g., less than 300° C.) when a diesel vehicle is driven in urban driving cycles. Thus, urea related deposits may be formed on the SCR catalyst. The urea related deposits may plug pores in the washcoat and reduce the catalyst surface area and the catalyst's activity. Further, clogging on the catalyst may increase the back pressure over the catalyst, and thus negatively impact engine performance and increase fuel consumption.
U.S. Pat. No. 6,892,530 discloses a method to regenerate a urea-based SCR catalyst by maintaining its temperature above the boiling point of hydrocarbons to remove hydrocarbon deposits. However, the inventors herein have recognized that the above approach may not remove urea related deposits sufficiently. For example, the temperature and time interval for removing hydrocarbon deposits used in the '530 patent may not be effective to remove the urea-based deposits.